Electroluminescence (EL) occurs by the emission of light from a phosphor in response to a sufficiently high electric field developed across the phosphor. Phosphor refers to those materials which emit light in response to the application of a field across the material. Thin film electroluminescent devices have a basic structure comprising a phosphor film or layer sandwiched between two electrodes. EL laminates are typically made by depositing the various layers onto a substrate such as quartz or glass, first a reflective metal layer onto the substrate onto which an insulating dielectric layer is deposited. The phosphor layer is then deposited onto the dielectric layer and then an optically transparent electrode, typically a transparent conducting oxide such as ITO is deposited onto the top surface of the phosphor layer. Application of an effective voltage between the two electrodes produces the electric field strength required to induce electroluminescence in the phosphor. The role of the dielectric layer is to reduce the voltage drop across the phosphor layer to avoid dielectric breakdown of the phosphor.
There is strong commercial interest to achieve the full spectral range in electroluminescent phosphors for visible display application and in particular for making colour flat panel displays. Sulphide phosphors are well known as efficient light emitters in electroluminescence as discussed in T. Inoguchi, M. Takeda, Y. Kakihara, Y. Nakata, M. Yoshida, SID'74 Digest, p. 84-85, 1974. These include ZnS:Mn and SrS:Ce. A significant drawback to these phosphors is that they are moisture sensitive and are prone to reacting with oxygen especially when electrically driven. Known electroluminescent materials being studied include materials such as SrS:RE, see W. A. Barrow, R. E. Coovert, C. N. King, Digest 1984 SID International Symposium, Los Angeles, p. 249, SrGa.sub.2 S.sub.4 :RE and CaGa.sub.2 S.sub.4 :RE as disclosed in W. A. Barrow, R. C. Coovert, E. Dickey, C. N. King, C. Laakso, S. S. Sun, R. T Tuenge, R. Wentross, Digest 1993 SID International Symposium, Seattle, p. 761; W. Halverson, T. Parodos, P. Colter, Display Phosphors Conference, San Diego, Nov. 13-16, 1995, p.115; S. S. Sun, E. Dickey, R. Tuenge, R. Wentross, Display Phosphors Conference, San Diego, Nov. 13-16, 1995, p.119; T. Yang, M. Chaichimansour, W. Park, B. K. Wagner, C. J. Summers, Display Phosphors Conference, San Diego, Nov. 13-16, 1995, p.123; and T. S. Moss, D. C. Smith, J. A Samuels, R. C. Dye, Display Phosphors Conference, San Diego, Nov. 13-16, 1995, p.127. While these materials do achieve red, green and blue emission, the gallium based sulphides suffer from low brightness, difficulty of preparation and stability problems.
It has recently been demonstrated that in the gallate based family of materials, ZnGa.sub.2 O.sub.4 :Mn could achieve bright and stable electroluminescence, see T. Minami, S. Takata, Y. Kuroi, T. Maeno, Digest 1995 SID International Symposium, Orlando, p. 724; and T. Minami, Y. Kuroi, S. Takata, Display Phosphors Conference, San Diego, Nov. 13-16, 1995, p.91. They obtained good green emission (200 cd/m.sup.2 at 60 Hz at up to 0.9 lm/w) but only obtained 0.5 cd/m.sup.2 blue, and 11.0 cd/m.sup.2 red at a drive frequency of 1000 Hz, which are not practical brightness values for a display by replacing Mn with Ce and Eu, respectively. They annealed these phosphor materials at 1020.degree. C. in argon.
More recently, Minami at al. have doped ZnGa.sub.2 O.sub.4 with chromium to generate a better red phosphor, claiming 120 cd/m.sup.2 at 1000 Hz, as disclosed in T. Minami, Y. Kuroi, S. Takata, T. Miyata, presented at Asia Display'95, Oct. 16-18, Hamamatsu. However it is not feasible, to achieve full colour in ZnGa.sub.2 O.sub.4 since rare earths are not compatible with this host lattice due to the size mismatch between Zn or Ga and the rare earth ions.
In the binary gallium oxide based system, .beta.-Ga.sub.2 O.sub.3 has the .theta.-alumina type structure with two different Ga sites, one in tetrahedral coordination and the other in octahedral coordination. High temperature heat treated .beta.-Ga.sub.2 O.sub.3 is known to exhibit bright broad-band photoluminescence under 254 nm UV irradiation and cathodoluminescence between 340 and 650 nm as disclosed in W. C. Herbert, H. B. Minnier and J. J. Brown, Jr., J. Electrochem. Soc. vol. 116, pp. 1019-1021(1969). .beta.-Ga.sub.2 O.sub.3 :Cr, the gallium analog of ruby, has been studied as a potential red to infrared tunable laser material because of the broad-band emission between 650 and 950 nm that is associated with the .sup.4 T.sub.2 -.sup.4 A.sub.2 transition of the Cr.sup.3+ ion in the octahedral site, see for example H. H. Tippins, Phys. Rev. vol. 137, pp. A865-A871(1965), and D. Vivien, B. Viana, A. Revcolevschi, J. D. Barrie, B. Dunn, P. Nelson and O. M. Stafsudd, J. Lum. vol. 39, pp. 29-33 (1987).
Even though there exists a significant difference in ionic radii of the rare earth ions and Ga.sup.3+, Ga.sub.2 O.sub.3 :Dy was reported to be a reasonably efficient photoluminescent phosphor with characteristic narrow Dy.sup.3+ lines in the blue (470-500 nm) and the yellow (570-600 nm) regions, see W. C. Herbert, H. B. Minnier and J. J. Brown, J. Electrochem. Soc. vol. 115, pp.104-105 (1968). Other common rare earth dopants such as Eu.sup.3+ and Tb.sup.3+, however, did not show efficient PL emission in .beta.-Ga.sub.2 O.sub.3, see J. L. Sommerdijk and A. Bril, J. Electrochem. Soc. vol 122, pp. 952-954 (1975). Sommerdijk also disclosed the solubility of Dy.sup.3+ in .beta.-Ga.sub.2 O.sub.3 to be only about 1%. W. C. Herbert, H. B. Minnier and J. J. Brown, J. Electrochem. Soc. vol. 115, pp.104-105 (1968) disclosed that maximum PL brightness occurred in the range of 5-10% (mole per cent) Dy concentration. The mechanism of the rare earth activation is still not clear.
Recently it has been demonstrated that Zn.sub.2 SiO.sub.4 :Mn could achieve electroluminescence, see T. Miyata, T. Minami, Y. Honda and S. Takata, SID '91 Digest, p. 286-289, 1991. Thin films were RF magnetron sputtered onto polished BaTiO.sub.3 substrates using the method disclosed in T. Minami, T. Miyata, S. Takata, I. Fukuda, SID'92 Digest, p. 162. A good brightness of 200 cd/m.sup.2 was achieved at 60 Hz with an efficiency of 0.8 lm/W. A drawback to these films is that they had to be annealed at 1000.degree. C. for several hours, which severely limits their applicability to practical substrates for displays.
As mentioned above, a major drawback to known electroluminescent materials is the need for post fabrication high temperature annealing (in the vicinity of 1000.degree. C.) of the films to produce electroluminescent behaviour. This need for high temperature treatment results in severe restrictions in the choice of substrates with only a limited number being available for use under these conditions. High temperature annealing also increases the cost of producing EL films rapidly on a large scale. Another limitation of many electroluminescent materials is that they are restricted to emitting at particular wavelengths or in a relatively narrow wavelength range, such as yellow ZnS:Mn or blue-green SrS:Ce which are not ideal for color displays that requires emission in the red, green and blue parts of the visible spectrum. Electroluminescent materials based on sulphides inherently suffer from chemical stability problems such as oxide formation (since oxides are generally thermodynamically more stable than sulphides) which changes the electronic properties of the material over time.
The classic EL phosphor, ZnS:Mn, is yellow and has a peak wavelength of 580 nm. However, while it may be filtered red and green, most of the light is lost because only .ltoreq.10% of the light is passed through the red and green filters. Similarly, a drawback of SrS:Ce, which is green-blue, is that only about 10% of the light is passed through a blue filter.
In the rare-earth doped oxides, narrow peaks that are red, green or blue result from dopants Eu.sup.+3 or Tb.sup.+3 so that little or no light is generated at wavelengths that are positioned in the visible spectrum sway from the desired red, green and blue wavelengths.
It would therefore be very advantageous to provide a method of producing new electroluminescent materials which can be deposited at temperatures well below 1000.degree. C. thereby avoiding the requirement for high temperature annealing. It would also be advantageous to provide new electroluminescent materials which emit over a broader portion of the visible spectrum than known EL materials. More specifically it would be very advantageous to provide a white phosphor, and phosphors with red, green and blue emission.
It would also be advantageous to provide a method of producing new EL materials that are chemically stable and do not react appreciably with water or oxygen and provide stable EL performance in which the brightness is maintained substantially constant during operation. Known color phosphors such as SrS:Ce and other sulphides are not stable in these respects.